The non-aqueous electrolyte secondary batteries, having a high capacity and a high energy density as compared with the conventional secondary batteries, are actively used for small-size electronic equipment, such as portable phones and notebook personal computers.
The non-aqueous electrolyte secondary batteries include a non-aqueous electrolyte, a positive electrode, and a negative electrode. The non-aqueous electrolyte generally comprises a non-aqueous solvent and a lithium salt dissolved therein. The positive electrode includes a positive electrode active material and a positive electrode current collector carrying the same. For the positive electrode active material, a material capable of reversibly absorbing and desorbing lithium ions and exhibiting a stable potential is preferably used. The negative electrode includes a negative electrode active material and a negative electrode current collector carrying the same. For the negative electrode active material, a material capable of reversibly absorbing and desorbing lithium ions and exhibiting a stable potential is preferably used. The positive electrode and the negative electrode have, for example, a sheet-like shape or a strip-like shape.
Between the positive electrode and the negative electrode, a separator made of an insulative porous material is interposed. An electrode assembly is fabricated, for example, by winding the positive electrode and the negative electrode with the separator interposed therebetween or stacking the positive electrode and the negative electrode with the separator interposed therebetween. The electrode assembly is housed together with the non-aqueous electrolyte, in a battery case of various shapes (a prismatic shape, a cylindrical shape, etc.), whereby a non-aqueous electrolyte secondary battery is fabricated.
In recent years, reduction in size and weight of electronic equipment has been advanced rapidly. Under such circumstances, the non-aqueous electrolyte secondary batteries to be used as a power source have been required to be further smaller in size and have a higher output than ever before. Accordingly, in light of improving the housing efficiency of the electrode assembly in the battery case, the reduction in thickness of the separator has been required.
Moreover, since the battery temperature rises when the positive electrode and the negative electrode are short-circuited, providing the separator with a function capable of ensuring safety is also required. The heat resistance of the separator is strongly dependent on its material. In view of this point, study has been made about a separator formed by stacking a layer made of an aramid resin excellent in heat resistance and a porous layer having a shutdown function, and other separators. The shutdown function is a function to cause melting when the battery generates heat, to close the pores, and block the migration of lithium ions. The porous layer having the shutdown function becomes a substantially non-porous layer at high temperatures.
Further, in view of maintaining the output performance of non-aqueous electrolyte secondary batteries, it is also important to ensure the lithium ion permeability of the separator.
As described above, the separator for use in a non-aqueous electrolyte secondary battery is required to satisfy a reduction in thickness, an ensured permeability of lithium ions, a function capable of ensuring safety at high temperatures, and the like.
Meanwhile, in recent years, a separator including an aramid resin, which is a heat resistant resin, has been actively developed.
For example, one proposal suggests a separator having a heat resistant layer including both a nitrogen-containing aromatic polymer, which is a heat resistant resin, and a ceramics powder. The heat resistant layer is disposed on the surface of a base material made of fibers, non-woven fabric, paper or porous film. The weight per unit area of the suggested base material is 40 g/m2 or less, and the thickness of the base material is 70 μm or less (see Patent Document 1). Patent Document 1 discloses a porous film made of polyethylene as an example of the base material.
Another proposal suggests forming an aramid resin film on the surface of a separator having the shutdown function (see Patent Document 2). Patent Document 2 discloses a separator comprising a porous film made of polyethylene (thickness: 16 μm) and an aramid resin film (thickness: 5 μm).    Patent Document 1: Japanese Patent Publication No. 3175730    Patent Document 2: Japanese Laid-Open Patent Publication No.